In recent years, from a perspective of preserving the environment, it is becoming popular to use a wind turbine generator which generates power from wind power. In a wind turbine generator, a kinetic energy in the wind is converted into a rotation energy of a blade (i.e. a rotor as a whole including the blade) and the rotation energy is converted into electric power in a generator.
Electricity generated by the wind turbine generator (wind turbine output) is expressed by Formula 1 below. The wind turbine output is improved with higher blade efficiency and a greater blade diameter.Wind turbine output=½*air density*(wind speed)3*blade efficiency*conversion efficiency*π*(blade diameter/2)2  (Formula 1)
The blade efficiency, herein, refers to an efficiency of converting the kinetic energy in the wind to the rotation energy of the blade (i.e. a rotor as a whole including the blade). The conversion efficiency refers to an efficiency of transmitting the rotation energy of the rotor to the generator and generating power in the generator.
It is know that blade efficiency has a maximum theoretical efficiency (Betz's limit=0.593). Specifically, even at the maximum theoretical efficiency, only 59.3% of the kinetic energy in wind can be converted to the rotation energy of the blade (precisely, the rotor as a whole, including the blades). In an actual wind turbine blade, the blade efficiency that can be achieved is up to approximately 0.5 due to influence of swirl of a wake and presence of air resistance.
Presently, the blade efficiency of the wind turbine blade developed for practical use is typically around 0.49. This leaves merely about 0.01 in the blade efficiency (2% of the entire blade efficiency of the existing blade) to improve by improving the blade design. However, the blade design improvement may lead to increased noise and efficiency decline due to off-design condition and thus may be undesirable.
Therefore, it is difficult to significantly increase the power generated by the wind turbine generator by improving the blade efficiency.
Meanwhile, the power generation output is affected as the square of the blade diameter. Thus, it is effective to increase the blade diameter so as to enhance the power generation output. However, the increased blade diameter leads to an increase in load acting on the wind turbine blade from the wind (aerodynamic load) and an increase in load (weight load) in response to increased weight of the wind turbine blade, which may result in a larger nacelle to support the rotor and in increased cost.
In view of this, it is possible with a modified design of an airfoil (a cross-sectional shape of the wind turbine blade) to reduce the load on the blade from the wind (aerodynamic load) by making the chord of the wind turbine blade shorter and also to suppress the increase in the load (weight load) in response to the increased weight of the wind turbine blade.
There are flatback airfoils with thicker trailing edge in a portion closer to a hub side of the wind turbine blade (e.g. Patent Literatures 1 through 5 and Non-Patent Literature 1).
Specifically, Patent Literature 1 describes a wind turbine blade having a plurality of flatback airfoils defined by coordinates. Patent Literature 2 proposes a flatback airfoil of a divergent type whose blade thickness around the trailing edge increases as closer with shorter distance to the trailing edge. A technique of producing a wind turbine blade having a flatback airfoil using a flatback airfoil insert is disclosed in Patent Literature 3. Patent Literature 4 discloses a wind turbine blade of a flatback airfoil type having a splitter plate attached to a trailing edge to reduce noise. Patent Literature 5 proposes to additionally attach a blade element to a wind turbine blade having a sharp trailing edge so that the airfoil profile is changed to a flatback airfoil profile by increasing a thickness of the trailing edge. Further, in Non-Patent Literature 1, evaluation results of aerodynamic characteristics of a flatback airfoil using several calculations are shown.
FIG. 15 shows a wind turbine having a flatback airfoil. The wind turbine blade 100 has a thickness in the trailing edge 8, which generates wake in a area 102 downstream of the trailing edge 8. A negative pressure in the area 102 downstream of the trailing edge 8 draws the flow of the air flowing along a suction-side surface 14 and delays separation of a boundary layer from the suction-side surface 14. More specifically, the negative pressure generated in the area 102 downstream of the trailing edge 8 draws the flow of the air toward the suction-side surface 14, thereby suppressing the separation of the boundary layer and moving a separation point of the boundary layer on the suction-side surface 14 toward the downstream side to a vicinity of the trailing edge 8 (fixing the separation point of the boundary layer to the vicinity of the trailing edge 8). Thus, in comparison to a conventional airfoil with a sharp trailing edge, this airfoil creates lift to a high angle of attack. Therefore, it is possible to obtain enough lift in spite of a shorter chord length. By reducing the chord length, the aerodynamic load acting on the wind turbine blade from the wind can be reduced.
The flatback airfoil has a blunt trailing edge and thus, section modulus of the flatback airfoil is superior to that of the conventional airfoil with a sharp trailing edge. Thus, it is possible to reduce the weight of the blade while maintaining its strength.
Although not related to a flatback airfoil, Patent Literature 6 discloses a wind turbine blade provided with an additional portion on a pressure-side near a trailing edge in a transition portion between an airfoil portion and a blade root portion to increase lift.